1. Field of the Invention
The present invention relates generally to methods for modifying the sucrose content of fruit. In particular, it relates to methods for increasing fruit sucrose content by inhibiting the expression of acid invertase. Additionally, it relates to methods for decreasing fruit sucrose content and increasing fruit hexose content by over expressing acid invertase.
A molecular genetic approach to altering sucrose accumulation in fruit is desirable because it is more precise and reproducible than using breeding plans. Breeding plans are laborious and require introduction of a trait found in wild type plants such as L. chmielewskii into non-wild plants such as L. esculentum. The desired trait may be linked to many undesired traits such as small, non-red, nonripening fruit and to a trait rendering the plant sterile. Also, the molecular genetic approach is applicable to many species for which gene transfer technology is available. Finally, cloning a gene permits elaboration of its end product in quantity.
2. Information Disclosure
Generally, a high soluble solids content is desirable in tomatoes because of its economic significance. Hewitt and Garrey, Tomato Biotechnology, 45-54 (Nevins and Jones, eds. 1987). Increased accumulation of sucrose contributes to elevation of the soluble solids content. U.S. patent application Ser. No. 509,673. The fruit of the commercially grown tomato cultivar, Lycopersicon esculentum, accumulate glucose and fructose, but not sucrose. Yelle et. al., Plant Physiol, 87:737-740 (1988). Breeding plans have produced at least one variety showing increased soluble solids. Rick, Hilgardia, 42:494-509 (1974). Work with one breeding plan resulted in the introduction of a genetic factor that confers on tomato plants the ability to bear mature fruit which accumulate sucrose, thereby increasing the total soluble solids content of the fruit. Patent application Ser. No. 509,673.
Generally, sucrose may be broken into the hexose sugars glucose and fructose by the enzyme invertase. Genetic and biochemical attributes of invertase have been studied. Yelle et. al., Plant Physiol. 95:1026-1035 (1991). Carrot acid invertase has been cloned. Sturm et. al., Plant Cell, 2:1107-1119 (1990). There are two regions of close similarity between genes encoding invertase and sucrose hydrolase. Sturm at 1115. The fruit of Lycopersicon chmielewskii, L. hirsutum. and L. peruviatum have been shown to accumulate sucrose instead of glucose and fructose. Davies, Nature, 209:640-641 (1966). The prior art does not teach the expression of tomato acid invertase in elevated levels in cell lines or manipulation of its activity using recombinant techniques.